The words mutual, mutuel sound the same but have different meanings and spellings. Why do mutual, mutuel sound the same even though they are completely different words?
The answer is simple: mutual, mutuel are homophones of the English language.
Having the same relationship each to the other: mutual predators.
Directed and received by each toward the other; reciprocal: mutual respect.
Possessed in common: mutual interests.
Of, relating to, or in the form of mutual insurance.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").